light sensitive line tracing apparatus



March 10, 1964 F. BROUWER 7 3,124,691

LIGHT SENSITIVE LINE TRAC ING APPARATUS Filed Sept. 19, 1960 4Sheets-Sheet 1 INVENTOR FRANS BROUWER ATTORNEY March 10, 1-964 F.BROQWER 3,124,691

LIGHT SENSITIVE LINE TRACING APPARATUS Filed Sept. 19, 1960 4Sheets-Sheet 3 FIG. 4,

INVENTOR FRANS BROUWER ATTORNEY March 10, 1-964 F. BROUWER LIGHTSENSITIVE LINE meme APPARATUS Filed Sept. 19, 1960 4 sheets sheet 4 FIG.8.

Va/IS- 4'0 line By ATTORNEY United States Patent 3,124,691 LIGHTSENSITIVE LINE TRACING APPARATUS Frans Brouwer, Glencoe, 11]., assignorto Stewart-Warner Corporation, Chicago, III., a corporation of VirginiaFiled Sept. 19, 1960, Ser. No. 56,920 20 Claims. (Cl. 250-202) Thisinvention relates to line sensing devices such as are useful inapparatus to control the patterns of torch cutters, wood sawingmachines, clothes cutters, welding tools, milling machines and the like.More particularly, this invention relates to guidance elements andsystems using photosensitive means of the general character shown anddescribed in U.S. Patent 2,489,305 to Miles A. McLellan, filed February12-, 1948, and issued November 29, 1949.

It is an object of this invention to provide a sensing element with avariable electric signal output dependent on the position of a sensedline.

It is also an object of this invention to provide a line sensing elementwhich has a simplified construction and has a minimum of moving partsmaking it economical to construct and to maintain.

Another object of this invention is to provide a sensing element whichis less sensitive to changes in the intensity of light impinging on thepattern to be traced.

It is another object of this invention to provide apparatus for guidingheavy or light machinery in accordance with a desired pattern drawn on amovable or stationary sheet of paper or the like.

It is also an object of this invention to provide a system for producingsignals which may be used to energize a servo drive mechanism capable ofguiding the apparatus and its corresponding operated mechanisms inaccordance with a desired complex pattern line even though the line mayclose upon itself.

Other objects and advantages will be obvious to one skilled in the artwith a further reading of this specification. These objects areaccomplished in an apparatus embodying the present invention whichcomprises photosensitive means responsive to variations in lightintensity for causing fluctuations in an electric current proportionalto the variations in light intensity upon which an image of the opticalfield including the line is projected. Opaque means are provided forperiodically obstructing the image of the field on the currentfluctuating means [for a portion of a cycle of oscillation. Furthermeans are provided which are responsive to the fluctuations in theelectric current to maintain the photosensitive means at 21 referenceposition with respect to the line to be traced.

This invention will be better understood upon a further reading of thespecification with reference to the attached drawings in which:

FIGURE 1 is an elevation view of a guidance system embodying thisinvention;

FIGURE 2 is a vertical section view of a sensing head for the guidancesystem of FIGURE 1;

FIGURE 3 is a bottom view of the photocell and vibrator assembly shownin FIGURE 4;

FIGURE 4 is a vertical section of the photocell and vibrator assemblytaken along line 4-4 of FIGURE 3;

FIGURE 5 is an enlarged detail view in perspective of the vibratorassembly;

FIGURE 6 is an enlarged fragmentary view in perspective of the adapterof the assembly shown in FIGURE 4;

FIGURE 7 is a schematic of the electrical circuitry of the guidancesystem;

FIGURE 8 is a graph showing the vibrator displacement and voltageoutputs of the photocell with respect to time, for one embodiment of theinvention;

FIGURE 8a is a schematic drawing showing the geo- 3,124,691 PatentedMar. 10, 1964 metric conditions for which the curves of FIGURE 8 arerepresentative;

FIGURE 9 is a graph showing the vibrator displacement and the voltageoutput of the photocell with respect to time, for another embodiment ofthe invention;

FIGURE 9a is a schematic drawing showing the geometric conditions forwhich the curves of FIGURE 9 are representative; and

FIGURE 10 is a schematic representation of the oscillations of theopaque shade with respect to the photocell which represents anotherembodiment of this invention.

For a description of one embodiment of the invention, reference is madeto FIGURE 1 which shows a guidance system 2% comprising a sensing head22 rotatably mounted in a frame 24. The sensing head 22 is caused torotate by a 2-phase servo steering motor 26 fixedly mounted on the frame24 and having a pinion gear 28 engaging a drive gear 30 on the sensinghead 22. The steering motor operates by an electric signal which variesin electrical properties in accordance with the position of the sensinghead over a line 31. A tachometer generator 33 is directly coupled tothe servo steering motor and serves a purpose which will be hereinafterdescribed. The steering motor 26 also rotates a drive mechanism 32 whichis rotatably mounted on the frame 24 by means of a gear 34 in engagementwith the pinion 28. The driving mechanism 32 has a wheel 36- which isdriven by a separate constant speed motor (not shown) within the drivemechanism. The wheel 36 is in friction engagement with the surface uponwhich is drawn the line 31 and serves to move the whole system 26 inaccordance with the signals of the sensing head 22. The guidance system20 is fixed- 1y connected to a work tool, such as a cutting torch 37, bymeans of an arm 38 attached to the frame 24. The torch 37 will,therefore, be caused to trace the same pattern the guidance system isfollowing to cut the desired shaped line in work piece 39.

The sensing head 22, which produces the fluctuations in the electriccurrent, comprises, as shown in FIGURE 2', a tubular housing 40 attachedto the frame 24 which has an aperture 42 therein abutting a shoulder 44formed in the housing and secured thereto by appropriate screws (notshown). A tube 48 is journaled within ball bearings 50 in the interiorof housing 40. The bearings 50 are secured by clip rings 52 withinannular grooves 54 in the bearings adjacent the ends of the housing 40.The gear 30, which is driven by the pinion 28, encircles the upper endof the tube 48 and is secured thereto by convenient means such as setscrew 58.

At the lower end of the tube 48 is positioned a lens assembly 6i) whichcomprises a lens "62 firmly fixed within a lens support 64. The lensassembly 60 is attached to a flange 66 at the lower end of the tube 48by screws 68 extending through elongated apertures 76 in the lenssupport and threadedly engage the flange 66. A small plate 72 which isfastened to a flattened portion 74 on the periphery of flange :66 has anadjusting screw 76 passing therethrough and engaging internal threads 78in the lens support 64 providing for radial adjustment of the lens 62with respect to the longitudinal axis of the tube 48.

A light source 80 is positioned at the lower end of the sensing elementhead 22 and is fastened to a collar 82 by a plurality of screws 84. Thecollar 82 has a flange 86 engaged by the bolts 46 which secure it andthe light source to the bottom end of the housing 40. The light source80 comprises a substantially conical shaped hollow member 88 having aplurality of wells 90 circumferentially spaced around the upper portion92 of the conical member 88 within which are disposed electric lamps 94.An equal plurality of passages 96 extend from the lower end of theconical member 88 and are aligned between the lamps 94 and a focal pointon the line 31 to be traced.

At the upper end of the sensing head is an assembly comprising aphotocell 102, a vibrator assembly 104, and a slip ring andelectromagnet subassembly 106, all supported on an adapter 108 fastenedto gear 30 by screws 110 extending through a flange 112 on the adapter.It will be noted that the assembly 100 is easily removable from the restof the sensing head making it readily accessible for maintenancepurposes.

The slip ring subassembly 106 comprises a cup-shaped member 116,fabricated of an insulating material, to which are attached four slipring commutators 118. The slip ring subassembly is secured to a tubularportion of the adapter 108 by screws 122 or other suitable fasteningmeans. The photocell 102 is rigidly secured within an aperture in a bar124 (FIG. 4) disposed within a groove 126 extending diametrically acrossthe bottom of adapter 108. The bar 124 is secured to the adapter 108 bymeans of screws passing through enlarged apertures 129 in the bar andthreadedly engaging the adapter. It will be seen in FIGURE 3 that thephotocell 102 has a substantially rectangular sensitive area 127 whichis aligned lengthwise with the longitudinal axis of the bar 124. Wires132 connect the photocell to slip rings 118a and 11012 which areconnected by brushes to a circuit to be hereinafter described. The lens62 near the bottom end of the sensing head 22 is adapted to cast animage of the scanning field 120 (FIG. 2) containing the line 31 on thesensitive area 127 of the photocell 102. The lens is adjusted to view asegment of the line which is displaced a short distance from the axis ofrotation of the sensing head. This gives the device its steering sensewhereby the steering motor rotates the driving mechanism and the sensinghead about their axis pursuant to a sensed transverse deviation of theimage of the line from a reference position. It is to be understood thata lens may not be necessary in the sensing head if the photocell ispositioned close enough to the line on the work surface. In this casethe photocell would be offset from the axis of rotation to give thedevice its steering sense. An annular light baflle is provided withinthe tube 48 to prevent unwanted light reflections from reaching thephotocell.

The vibrator subassembly 104 comprises an arm 134 having a right angleprojection 136 (FIG. 2) forming an opaque shade adjacent to thesensitive area 127 on the photocell 102 between it and the lens 62. Theopaque shade extends in a direction across the short dimension of therectangular sensitive area 127 (FIG. 3). The

arm 134- has a U-shaped fold 140 (FIG. 5) at its other end in the sameplane as the opaque shade 136. There is a slot 142 (FIG. 4) in thefolded end of the arm which extends from the base 144 through a portionof each of the legs 146 of the U-shaped fold. A flat spring 148 isinserted in the slot 142 forming an aperture 150 with the base and twolegs of the U-shaped fold. A U-shaped permanent magnet 152 has its base154 disposed within said aperture 150 and has its legs 155 extending ina direction opposite from the opaque shade end of the arm 134. The arm134, flat spring 148 and magnet 152 are suitably secured together toform a rigid assembly by means of an epoxy resin or other suitablefastening means.

The ends 156 of flat springs 148 are received in diametrically opposedslots 158 (FIG. 6) at the upper end of the tubular portion 120 ofadapter 108. There is a second pair of slots 160 immediately adjacentthe spring containing slots 160 forming a tongue 162 therebetween. Awedge (not shown) is driven into each of the slots 160 forcing thetongues 162 against the spring ends 156 securing them in rigidengagement within the slots 158. Thus the arm 134 may be caused toangularly vibrate about an axis described by the center of the flatspring 148 to cause the opaque shade 136 to periodically trava erse theimage of the scanning field projected on the sensitive area 127 of thephotocell 102.

To vibrate the arm 134 the permanent magnet 152 is subjected to afluctuating magnetic field established by an electromagnet 164 (FIGS. 2and 4) which is connected to an alternating current source through wires165, and slip rings 1180 and d. The electromagnet 164 has an E-shapedcore 166 having legs 167a and b extending towards the legs 155 of theU-shaped permanent ma net 152. A coil 168 is wound around the center leg16711 of the core 166 which causes a fluctuating magnetic field to beestablished through the outer legs 167b causing the permanent magnet152, and hence the arm 134, to vibrate in accordance with theoscillations of the energizing current. The electromagnet is implantedin an epoxy resin 169 or the like within the cup-shaped member 116 tohold it firmly therein.

The vibrating assembly is adjusted so that the shade 136 on the arm 134-vibrates with an equal amplitude on either side of a plane which isperpendicular to the sensitive area and includes the center of thescanning field 128. The arm vibrates essentially within a plane which isparallel to the longer dimension of the sensitive area. It will be notedin this particular embodiment that the axis of rotation of the opaqueshade is parallel to the plane of the sensitive area. Otherconfigurations are possible, however, as will be described hereinafter.

The opaque shade will align itself generally with the image of the line31 being traced. The variations in the position of the line with respectto the center of vibration causes changes in the electrical signaloutput of the photocell which directs the steering motor to turn thesensing head toward the line as hereinafter described. The enlargedholes 129 in the bar 124 permit adjustment of the position of thephotocell with respect to the vibrating arm to compensate for anyirregularities in the photocell to obtain a properly balanced signal.

The photocell 102 may be of any well known type with appropriatecircuitry. FIGURE 7 shows a circuit usable with a photoresistive-typecell whose resistance varies inversely with light intensity on itssensitive area. The photocell 102 is connected in series with a resistor170 across a negative DC. voltage. The resistor 172 shown in phantom isnot actually included in the circuit but is shown to help describe acertain effect as will be hereinafter described. The vibratorsubassembly 104 is schematically shown with the shutter 136 operable tovibrate over the sensitive area 127 of the photocell 102 in response toan alternating current signal applied through transformer 174. A phaseadjusting circuit 175 is included in the circuit energizing the vibratorsubassembly to compensate for phase shifts in various parts of theelectrical circuit. The phase adjusting circuit is not shown in detailas it may be any well known type and does not form a part of thisinvention.

FIGURE 8 shows an example of the kind of signals that may be derivedfrom the output of the photocell connected in a circuit as shown inFIGURE 7. The curves represent voltage signals at the point 178 at theoutput of the photocell. In this case it is assumed that the width ofthe projected line 31 is approximately equal to the width of the shade136 and that the width of the sensitive area 127 of the photocell 102 islarger than three times the width of the shade 136 as shown in FIGURE8a. The width of vibration is somewhat but not much smaller than thelongitudinal dimension of the sensitive area 127. The top curve a inFIGURE 8 shows the shutter amplitude versus time and the relationship issinusoidal. Curves b, c, d and e are voltage versus timerepresentations. Curve b shows the signal on the cell 102 when theprojection of the line 31 is central which coincides with the shade 136in the condition when the vibrator is not operating. Curves c, d and eshow the signals when the line projection 31 is off center byrespectively A, /2 and 1 width of the shade 136, as seen in FIG- URE 8a.

These graphs may readily be derived by considering the geometry of thisexample. For example, in curve b of FIGURE 8 representing the case wherethe line 31 is located centrally beneath the sensitive area 127, thelight intensity impinging on the sensitive area 127 will be greatestwhen the shade 136 is also centrally located beneath sensitive area 127.The resistance of the photocell 102 will have its smallest value and thevoltage at point 128 will swing towards the ground potential. As theshade 136 moves away from the centermost position, it blocks out anincreased portion of the light impinging on the area 127 causing theresistance of photocell 102 to increase, and, hence, cause the voltageat point 178 to swing more negative.

If the frequency of vibration is f, we see that the fundamentalcomponent in signal b is 2 However, curves c, d and e exhibit increasingcontent of the original frequency f. This component is in phase with thevibration frequency when the deviation of the projection from the centeris in the direction shown in FIGURE 8a and in the opposite phase whenthe deviation is on the opposite side of the center. When the line isabsent, no signal appears at the output of the photocell because thelight intensity appearing on the sensitive area is constant.

Hence, from the photocell signals we may derive information on whichside and how much oif center the deviation is by observing the phase andmagnitude of the frequency f signal. In addition, the presence orabsence of a signal indicates whether the photocell is above a line ornot.

FIGURE 9 shows an example of a ditferent configuration in which thewidth of the sensitive area 127 of the photocell 102 is approximatelyequal to the width of the projected line 31 and the shade 136 as shownin FIGURE 90. The curve a in FIGURE 9 again represents the shutteramplitude versus time and the curves b, c and d represent the samerespective deviations as the like lettered curves in FIGURE 8. It shouldbe noted that the indications of the above example are changed inmeaning in this case. That is, absence of any signal now indicates thatthe system is on the line. The presence of the 2 component at itsfullest magnitude indicates that there is no line in the scanning field.

The fluctuations in the output of the photocell may be passed through anelectric circuit and the information derived therefrom made to controlthe steering motor to make the photocell follow the line.

With reference again to FIGURE 7, the output of the photocell 102 is fedthrough two series connected A.C. amplifiers 180 and 182, respectively,to a second harmonic rejector circuit 184, which may take the form of aseries resonant circuit to shunt the second harmonic component of thesignal to ground. The remainder of the signal is amplified by amplifier186 and applied to one Winding 188 of the two-phase steering motor 26.The other winding 190 of the motor 26 is connected to the same A.C.source energizing vibrator 104 through a capacitor 192. The capacitorprovides a 90 phase shift required to properly phase the signals in thewindings of the two-phase type motor. The application of a signalcorresponding to the phase represented in FIGURE 8 will cause the motor26 to rotate in one direction and a signal of opposite phase will causethe motor to rotate in the opposite direction. When the photocell isdirectly over the line, there is essentially no phase shift as theoutput therefrom is virtually made up of even harmonics of the vibrationfrequency and thus causes no rotation of the motor.

The tachometer generator 34 is directly connected to the shaft of themotor 26 and has one winding 200 connected to the A.C. source. The otherwinding 202 is connected to the input of amplifier 186 and provides asignal thereto corresponding to the speed of rotation of the motor 26.This signal is in opposition to the input from the preceding circuitleading from the photocell 102. When there is a large deviation of theline 31 from the center of the sensitive area 127, a large outputresults from the photocell and the amplifiers tend to produce a largeoutput to drive the steering motor at a high speed. However, as thevelocity of the motor 26 increases the output from the tachometergenerator 34 increases and opposes the signal from the photocell thusreducing the output from the amplifier 186. This eflect permits the useof large amplification in the servo loop and reduces overshoot since themaximum amplification of the loop only becomes effective when the speedof the steering motor is near 0.

An alarm circuit 204 is connected to the output of amplifier andprovides a warning when there is no line in the scanning field imageprojected on the photocell. It also turns off the A.C. power to the feedmotor 205 which drives the wheel 36 in the driving mechanism 32. Thealternating current signal from the amplifier is converted to directcurrent by a conventional rectifier 266 whose output is connectedthrough amplifier 2% to the energizing coil 210 of relay 212. The coil210, when energized, operates the contacts 194 and 196. The contacts 194in series with the feed motor are normally open. Contacts 196, which areserially connected with lamp 1% across the A.C. feed source, arenormally closed. Thus, when the photocell is over a line a signalappears at the output of amplifier 180 which is rectified and amplifiedto energize the coil 210 of relay 212 causing normally open contacts 194to close and normally closed contacts 196 to open. The feed motor istherefore connected directly to the A.C. source and the circuit to thelamp 193 is opened. When, however, the apparatus encounters adiscontinuity in the line, there will be no signal at the output ofamplifier 130, and the relay 212 will deenergize. Contacts 194,therefore, open to disconnect the feed motor from the A.C. source, andcontacts 196 to close to connect the lamp 198 across the A.C. source towarn the operator that the photocell is no longer receiving an image ofa line.

The particular warning circuit 2% shown in FIGURE 7 presumes, of course,a configuration of the photocell and vibrator shown and described inFIGURES 8 and 8a. If the configuration of FIGURES 9 and 9a is used, thealarm circuit must be modified to cause the power to the motor to be cutoh and the warning light energized when the signal comprises a maximum2]- component. When there is no second harmonic component at the outputof the amplifier 130 it is indicative that the photocell is over a line,power must be on with the warning light shut off.

It is to be understood that many other modifications may be made to theparticular embodiment hereinbefore described. For example, the vibratorassembly might be of a type, as shown in FIGURE 10, in which the opaqueshade 136 angularly vibrates about an axis 22% which is perpendicular tothe plane of the sensitive area 127 of the photocell, and with equalamplitude on either side of a plane perpendicular to the plane of thesensitive area and intersecting the center of the scanned field. As longas the vibrations are of equal amplitude on either side of the centerplane, the angular vibration has no adverse effect on the signalproduced by the photocell as it encounters the line.

It has also been found that the operation of the device may be improvedwhen the scanning headis provided with a photocell having a relativelylong photosensitive area compared to the peak to peak amplitude of theopaque shade vibrations. A relatively large area of the photocell willthen be illuminated without interruption. This area of the photocelldoes not contribute to the signal but forms a resistive load 011 thecircuit which may be thought of as a variable resistance in parallelwith the photocell 1&2. The resistor 172, shown in phantom in FIGURE 5,represents this resistive load. When the light level of the image isreduced, the sensitivity of the photocell naturally drops. However, theefiective shunting resistance, represented as resistance 172, loadingthe circuit is increased so that it counteracts the reduction insensitivity. It has been found that the sensitivity of a photo elementhaving a relatively wide sensitive area is almost independent of smallvariation of the voltage of the light source.

It is to be understood that the sensing head hereinbefore described isnot limited to use with line tracing systems but may be used in manyother applications wherein it is necessary to determine the position ofa line. For instance, it may be useful as a position determining elementin precision gauging equipment such as line standard comparators. Itcould also be adapted for use as a sensing element in wire gauginginstruments or for remote observation of the relative position ofmachine tool elements with respect to a measuring scale. It is,therefore, intended that the invention should not be limited to theembodiments hcreinbefore disclosed, but should embrace all of theembodiments which may be included within the framework of the appendedclaims.

What is claimed is:

1. A head for a system for scanning a line comprising photosensitivemeans having a sensitive area positioned to have an image of thescanning field containing the line cast thereon, the length of thesensitive area in a direction transverse to the line being equal to orgreater than the width of the image of the line, continuous opaque meansdisposed between the sensitive area and the line, and means foroscillating said opaque means essentially transversely to the line forperiodically obstructing the image of the line on the sensitive area,said opaque means oscillating with equal amplitude across a planeperpendicular to the sensitive area and containing the center of thescanning field.

2. A head for a system for scanning a line comprising photosensitivemeans having an elongated sensitive area positioned to have an image ofthe line cast on said sensitive area transverse to the areas longestdimension, totally opaque means disposed between said sensitive area andthe line, and means for angularly oscillating said opaque meanstransversely to the line for periodically obstructing the image on thephotosensitive means, the axis of the angular oscillation beingpositioned within a plane perpendicular to and intersecting saidsensitive area.

3. The head of claim 2 wherein the axis of angular oscillation is alsoparallel to the plane of said sensitive area.

4. The head of claim 2 wherein the axis of angular oscillation is alsoperpendicular to the plane of said sensitive area.

5. The head as claimed in claim 2 wherein the longest dimension of saidelongated sensitive area is approximately equal to or greater than thewidth of the image of the line.

6. A scanning head for a system for tracing a line comprising aphotocell having an elongated rectangular sensitive area, said photocellpositioned to have an image of the scanning field containing said linecast upon said sensitive area, an arm having a continuous opaque portionpositioned between said sensitive area and the scanning field andextending transversely across said rectangular sensitive area, saidopaque portion having a width approximately equal to the image of thescanned line, and means for vibrating said arm to cause said opaqueportion to oscillate in a direction along the longer dimension of therectangular sensitive area.

7. A head as claimed in claim 6 wherein said opaque portion angularlyoscillates about an axis positioned in a plane perpendicular to andintersecting the plane of said sensitive aera.

8. A head as claimed in claim 7 wherein said axis is also parallel tosaid sensitive area.

9. A head as claimed in claim 7 wherein said axis is also perpendicularto said sensitive area.

10. The head as claimed in claim 6 wherein the longest dimension of saidrectangular field is approximately equal to or greater than the width ofthe image of the line.

11. A system for scanning a line comprising a scanning head, saidscanning head comprising photosensitive means having a sensitive areapositioned to have an image of the scanning field containing the linecast thereon, said photosensitive means adapted to cause fluctuations inan electric signal proportional to variations in light intensity, thelength of the sensitive area in a direction transverse to the line beingsubstantially greater than the width of the image of the line, opaquemeans disposed between the sensitive area and the line, means foroscillating said opaque means essentially transversely to the line forperiodically obstructing the image of the line on the sensitive area,said opaque means oscillating with equal amplitude across a planeperpendicular to said sensitive area and containing the center of thescanning field, means for selecting from the fluctuating signal outputof the scanning head an odd harmonic component of the oscillationfrequency, said odd harmonic component having the property of reversingits phase as the line moves from one side of said plane to the other,and means synchronized with the oscillations of said opaque means andrecipient of said odd harmonic component for producing differentelectrical outputs for the two opposite phases.

12. The system as claimed in claim 11 comprising in addition means forselecting from the fluctuating output of the scanning head an evenharmonic component of the oscillation frequency, and means in receipt ofsaid even harmonic signal for producing another electrical output,whereby the absence of receipt of said even harmonic signal indicatesthe absence of a line within the scanning field.

13. A system for scanning a line comprising a scanning head, saidscanning head comprising photosensitive means having a sensitive areapositioned to have an image of the scanning field containing the linecast thereon, said photosensitive means adapted to cause fluctuations inan electric signal proportional to variations in light intensity, thelength of the sensitive area in a direction transverse to the line beingapproximately equal to the width of the image of the line, opaque meansdisposed between the sensitive area and the line, means for oscillatingsaid opaque means essentially transversely to the line for periodicallyobstructing the image of the line on the sensitive area, said opaquemeans oscillating with equal amplitude across a plane perpendicular tosaid sensitive area and containing the center of the scanning field,means for selecting from the fluctuating signal output of the scanninghead an odd harmonic component of the oscillation frequency, said oddharmonic component having the property of reversing its phase as theline moves from one side of said plane to the other, and means synchronized with the oscillations of said opaque means and recipient of saidodd harmonic component for producing different electrical outputs forthe two opposite phases.

14. The system as claimed in claim 13 comprising in addition means forselecting from the fluctuating output of the scanning head an evenharmonic component of the oscillation frequency, and means in receipt ofsaid even harmonic signal for producing another electrical outputwhereby the receipt of said even harmonic signal indicates the absenceof a line within the scanning field.

15. A head as claimed in claim 1 wherein said oscillating meanscomprises an arm having a right angle projection forming said opaquemeans at one end thereof, a U-shaped fold at the other end of said armin the same plane as said right angle projection, said arm having a slotextending from the base of said U-shaped fold through a portion of eachleg of said U-shaped fold, a flat spring received in said slot andforming an aperture with th base and legs of said U-shaped fold; aU-shaped permanent magnet having its base received in said aperture,means for fastening said arm, flat spring and magnet rigidly together,an E-shaped electromagnet rigidly mounted with respect to said permanentmagnet and magnetically coupled thereto, and means for rigidly fasteningthe ends of said flat spring.

16. The head as claimed in claim 15 wherein said last mentioned meanscomprises a tubular member having a pair of oppositely disposed slots atone end, said slots receiving the ends of said flat spring, said tubularmember having a second pair of oppositely disposed slots at its one endimmediately adjacent to said first pair of slots forming a pair oftongues therebetween, and Wedge means within said second pair of slotsto force said tongues tightly against the ends of the springs.

17. A vibrating assembly for use with a scanning head in a line tracingsystem comprising photoelectric means positioned to have an image of theline cast thereupon, and opaque means driven by said vibrating assemblyfor periodically obstructing the image of the line upon saidphotoelectric means; said vibrating means comprising an arm having aright angle projection forming said opaque means at one end thereof, aU-shaped fold at the other end of said arm in the same plane as saidright angle projection, said arm having a slot extending from the baseof said U-shaped fold through a portion of each leg of said U-shapedfold, a flat spring received in said slot forming an aperture with thebase and legs of said U-shaped fold, a U-shaped permanent magnet havingits base received in said aperture, means for fastening said arm, fiatspring and magnet rigidly together, an E-shaped electromagnet rigidlymounted with respect to said perma nent magnet and magnetically coupledthereto and means for rigidly fastening the ends of said flat spring.

18. The vibrating assembly as claimed in claim 17 wherein said lastmentioned means comprises a tubular member having a pair of oppositelydisposed slots at one end, said slots receiving the ends of said flatspring, said tubular member having a second pair of oppositely disposedslots at its one end immediately adjacent to said first pair of slotsforming a pair of tongues therebetween and wedge means within saidsecond pair of slots to force said tongues tightly against the ends ofthe springs.

19. The vibrating assembly as claimed in claim 18 comprising in additiona bar adjustably mounted across the other end of said tubular member andrigidly supporting said photoelectric means within operable proximity ofsaid right angle projection.

20. A head for a system for scanning a line comprising photosensitivemeans having a sensitive area positioned to have an image of thescanning field containing the line cast thereon, the length of thesensitive area in a direc tion transverse to the line being equal to orgreater than the width of the image of the line, continuous opaque meansdisposed between the sensitive area and the line, and means foroscillating said opaque means essentially transversely to the line forperiodically obstructing the image of the line on the sensitive area.

References Cited in the file of this patent UNITED STATES PATENTS1,631,021 Dowling May 31, 1927 2,208,420} Gulliksen July 16, 19402,489,305 McLenn-an Nov. 29, 1949 2,791,696 Schell May 7, 1957 2,795,736Dilks June 11, 1957 2,816,283 Steele Dec. 10, 1957 2,899,564 Rabinow etal Aug. 11, 1959 3,017,801 Ingber Jan. 23, 1962 3,037,888 Lobosco et a1June 5, 1962

11. A SYSTEM FOR SCANNING A LINE COMPRISING A SCANNING HEAD, SAIDSCANNING HEAD COMPRISING PHOTOSENSITIVE MEANS HAVING A SENSITIVE AREAPOSITIONED TO HAVE AN IMAGE OF THE SCANNING FIELD CONTAINING THE LINECAST THEREON, SAID PHOTOSENSITIVE MEANS ADAPTED TO CAUSE FLUCTUATIONS INAN ELECTRIC SIGNAL PROPORTIONAL TO VARIATIONS IN LIGHT INTENSITY, THELENGTH OF THE SENSITIVE AREA IN A DIRECTION TRANSVERSE TO THE LINE BEINGSUBSTANTIALLY GREATER THAN THE WIDTH OF THE IMAGE OF THE LINE, OPAQUEMEANS DISPOSED BETWEEN THE SENSITIVE AREA AND THE LINE, MEANS FOROSCILLATING SAID OPAQUE MEANS ESSENTIALLY TRANSVERSELY TO THE LINE FORPERIODICALLY OBSTRUCTING THE IMAGE OF THE LINE ON THE SENSITIVE AREA,SAID OPAQUE MEANS OSCILLATING WITH EQUAL AMPLITUDE ACROSS A PLANEPERPENDICULAR TO SAID SENSITIVE AREA AND CONTAINING THE CENTER OF THESCANNING FIELD, MEANS FOR SELECTING FROM THE FLUCTUATING SIGNAL OUTPUTOF THE SCANNING HEAD AN ODD HARMONIC COMPONENT OF THE OSCILLATIONFREQUENCY, SAID ODD HARMONIC COMPONENT HAVING THE PROPERTY OF REVERSINGITS PHASE AS THE LINE MOVES FROM ONE SIDE OF SAID PLANE TO THE OTHER,AND MEANS SYNCHRONIZED WITH THE OSCILLATIONS OF SAID OPAQUE MEANS ANDRECIPIENT OF SAID ODD HARMONIC COMPONENT FOR PRODUCING DIFFERENTELECTRICAL OUTPUTS FOR THE TWO OPPOSITE PHASES.